1. Field of Invention
The present invention relates to the catalysts used in the deep catalytic cracking (DCC) of petroleum naphthas and other hydrocarbon feedstocks. More specifically, the invention provides catalysts containing silicon, aluminum, chromium, and optionally, monovalent alkaline metal oxides. Such catalyst compositions are capable of selectively converting petroleum naphthas and other hydrocarbon feedstocks into commercial valuable light olefins, mainly ethylene and propylene.
2. The Prior Art
It is known to use the technique of steam-cracking on light paraffins (ethane, propane and butane, obtained mainly by extraction from various natural gas sources) and on naphthas and other heavier petroleum cuts, to produce:    i) primarily ethylene and propylene;    ii) secondarily, depending on the feedstock employed, a C4 cut rich in butadienes and a C5+ cut with a high content of aromatics, particularly benzene;    iii) and finally hydrogen.
The feedstocks of choice are ethane and liquid petroleum gas (LPG) for the U.S.A. and naphthas and gas oils for Europe. However, in recent years, the situation has dramatically changed with the U.S.A. moving towards the utilization of heavier hydrocarbon feedstocks like in Europe.
It is worth noting that steam cracking is one of the core processes in the petrochemical industry with a worldwide production of ca. 100 million metric tons/year of ethylene and propylene.
Steam cracking is a thermal cracking reaction performed at high temperatures and in the presence of steam, a diluant which is concurrently fed with the hydrocarbon stream in a steam cracking reactor. The reaction temperature ranges from 700° C. to 900° C. according to the type of feedstock treated (the longer the hydrocarbon molecular structure, the lower the temperature of cracking), while the residence time ranges from a few seconds to a fraction of second.
Steam cracking is a well-established technology. However, it suffers from many drawbacks:    i) lack of flexibility in the product selectivity, mostly in the yield of propylene which needs to be increased in order to respond to the increasing demand of the market.    ii) significant production of fuel oil which contains heavy hydrocarbons such as heavy alkylaromatics and even polyalkylaromatics. It is known that the latter products are precursors of “coke”. Coking is a serious problem in the steam cracking technology, which decreases the energy efficiency and requires difficult de-coking procedures for reactors.    iii) in order to achieve a satisfactory conversion, severe operating conditions are used; i.e. high reaction temperatures and the recycling of gaseous paraffinic products.
More than twelve years ago, a process aiming at upgrading the products of propane steam cracking was developed in the laboratory of the present inventor [1]. The upgrading consisted of adding a small catalytic reactor to a conventional propane steam cracker. The catalysts used in the catalytic reactor were based on the ZSM-5 zeolite modified with Al and Cr [2]. Significant increases in the yield of ethylene and aromatics were obtained.
More recently, the present inventor's research group developed a further refined process [3,4] consisting of using two reactors in sequence, the first reactor (I) containing a mildly active but robust catalyst and the second reactor (II) being loaded with a ZSM5 zeolite based catalyst, preferably of the hybrid configuration. Hybrid configuration means that at least two co-catalysts are commingled. Variations of the temperature of reactor I versus reactor II and the textural properties and/or the surface composition of the catalyst of reactor (I) were used to increase the conversion and to vary the product distribution, namely the ethylene/propylene ratio.
Although our previous work is of great industrial interest, the use of two reactors, which require heating at different temperatures, represents a significant challenge in terms of technology and investment.
Thus, the present invention responds to the need for a simplified technology while maintaining catalyst performance and product flexibility at significantly higher levels than what is currently achieved with conventional steam cracking processes. The present invention focuses primarily on catalyst formulations.
Thus, it is an object of the present invention to provide novel catalysts for selective deep catalytic cracking (DCC) of petroleum naphthas and other hydrocarbon feedstocks.